1. Field of the Invention
The present invention relates to fabrication of electrical circuits, and more particularly concerns fabrication of circuits employing mandrels and semi-additive processes for making circuits having raised features.
2. Description of Related Art
In a prior application of William R. Crumly, Christopher M. Schreiber and Haim Feigenbaum for Three-Dimensional Electroformed Circuitry, Ser. No. 07/580,758, filed Sep. 11, 1990, assigned to the assignee of the present application, there are described techniques for fully additive forming of electrical circuits having integral raised features. The disclosure of this prior application is incorporated by this reference as though fully set forth herein. In general, the processes described in the prior application include the use of a stainless steel mandrel in which are formed raised features projecting from the mandrel surface, either downwardly into the mandrel or upstanding from the mandrel surface. A pattern of grooves in the form of a negative of a circuit to be fabricated is formed in the surface of the mandrel and filled with Teflon or other non-conductive material and the desired pattern of circuit traces is then electrodeposited onto the mandrel surface between the areas of Teflon and into the depressions or over the post of the three dimensional mandrel features. The use of Teflon embedded in groves within the mandrel to define the circuit traces limits the minimum size of circuit traces that can be formed, because, as the circuit trace is additively formed, as by electroplating, a plated material plates up laterally on either side at the same time that it plates up vertically above the surface. Accordingly, for example, to obtain a trace 1 mil thick and 4 mils wide, the conductive pattern line width on the mandrel can be no more than 2 mils in width, because the plating of a mil thickness also plates outwardly on either side to one 1 mil. Thus, fabrication of very fine circuit trace lines is limited.
If the Teflon pattern is deposited on the surface of the stainless steel mandrel instead of in grooves, finer circuit trace features and widths can be obtained, but difficulties are encountered in treating the mandrel surface to achieve satisfactory adherence of the Teflon to the stainless steel mandrel surface.
The fully additive techniques described in the above-identified patent application have many advantages. After the mandrel has been made there is no need to do any further printing or photo operations. All registration of the raised features with the circuit traces is completed, and there are no subsequent registration problems. Further, there are no further photolithographic operations, and much manual labor in the use of the completed mandrels for production of circuits is eliminated. Nevertheless the mandrel itself is relatively complex and expensive and time consuming to produce, and, therefore, finds maximum utility in relatively long production runs where rapid fabrication of the first prototype and production items is not essential. Thus, the fully additive mandrel for making circuits with raised features is most useful for long production runs where time of initial production is of lesser importance, but such mandrels are not as readily applicable for production of raised feature circuits in smaller quantities where the first prototype must be available in a short time.
Accordingly, it is an object of the present invention to provide for production of electrical circuits with raised features by methods that avoid or minimize above mentioned problems.